The present invention relates to optical communication terminals. In particular, the present invention relates to an optical communication terminal that includes a telescope shared by both transmitted and received optical energy.
Ground based communications systems commonly transmit and receive signals at radio frequency (RF) wavelengths. RF transmissions are suitable for ground systems due to the 10 relative insensitivity of RF signals to atmospheric effects. Atmospheric effects, which include rain attenuation and scintillation, for example, would significantly attenuate optical transmissions.
Furthermore, the well established and well understood nature of RF communications has made it the prevalent communication technology for space based systems. From its inception, satellite technology has employed RF communications. In space, or any other environment that is free of atmospheric effects, however, optical communications techniques may be employed to transmit and receive information at data rates far greater than those of RF communications.
The preference for RF communications, unfortunately, has resulted in less emphasis being placed on design and development of optical communication systems. Thus, in the past, few optical communications systems have been designed and fewer have been implemented on the ground. No commerical optical communication systems currently fly in space.
One previous conceptual approach to implementing an optical communication system centers around an optical communication terminal (xe2x80x9cterminalxe2x80x9d) with two optical communication paths, a transmit path and a receive path. The transmit path provides a physical path for transmit optical energy, while the receive path provides a physical path for received optical energy. The optical energy propagates along the transmit path and leaves the terminal through a transmit aperture, while the optical energy that propagates along the receive path first enters the terminal through a receive aperture separate from the transmit aperture.
Typically, a set of optics is required for the receive aperture to focus and direct the received optical energy into the receive path. An additional set of optics is required for the transmit aperture to expand and direct the transmitted optical energy outward through the transmit aperture. The transmit optics and receive optics are mounted, typically, in a structure and the combination is commonly referred to as a telescope. The opening in the telescope through which a transmitted beam leaves, or through which a received beam enters is commonly referred to as the transmit aperture and receive aperture, respectively.
Implementing a terminal with separate transmit and receive apertures has several drawbacks however. One drawback is that the mechanical complexity, cost, and size of the terminal are increased due to the need for separate transmit and receive telescopes, separate receive and transmit paths, and separate sets of optics for the transmit aperture and the receive aperture.
Another drawback is that the transmit telescope and the receive telescope separately experience mechanical stresses. Mechanical stresses on the transmit telescope and the receive telescope may, for example, cause misalignment of the transmit aperture independently of any misalignment of the receive aperture. Misalignment in the transmit aperture, in turn, may cause the terminal to send the transmit beam in a direction not aligned to the destination terminal. Additionally, misalignment in the receive aperture may result in the terminal communication detector receiving less or no optical energy from other transmitting terminals.
As a result, the conceptual design for an optical communication terminal using separate transmit and receive telescopes requires additional hardware to maintain very accurate co-alignment between the transmit and the receive telescopes. Maintaining co-alignment is a particular problem in space based systems which operate over extreme ranges of temperature. Thus, in the past, the initial concepts for optical communication terminals have been unduly complicated, bulky and costly, as well as mechanically unstable.
An additional design problem that must be addressed is the initial alignment (also referred to as acquisition) of individual terminals so that they may communicate with one another. Acquisition is an important, non-trivial problem because the transmit beam is typically extremely narrow. Even very small misalignments between a transmitting terminal and a receiving terminal may result in the transmit beam completely missing the receiving terminal.
One conceptual approach to initial alignment uses a beacon transmitter that independently emits a beacon beam of additional energy that is much broader than the transmit beam. The broader beacon beam is then received by a separate telescope on each terminal. The separate telescope includes optics which focus and direct the beacon beam into the terminal for acquisition purposes. This approach shares the disadvantages of the conceptual terminal design, however, in that the separate telescope and associated optics for the beacon beam increase the size, cost, and complexity of the terminal.
A need has long existed in the industry for an optical communication terminal which overcomes the disadvantages cited above and previously experienced.
It is an object of the present invention to provide an optical communication terminal.
It is another object of the present invention to provide an optical communication terminal having shared transmit and receive optical elements.
Yet another object of the present invention is to share a single telescope for both a transmit beam and a receive beam in an optical communication terminal.
Another object of the present invention is to provide an optical communication terminal that shares a single telescope between the transmit beam, the receive beam, and the acquisition beacon beam received.
Yet another object of the present invention is to use an annular mirror in an optical communication terminal to allow a shared transmit and receive path and a shared telescope in the optical communication terminal.
A still further object of the present invention is to allow an optical communication terminal to use a transmit beam that is the same wavelength as the receive beam.
Another object of the present invention is to allow an optical communication terminal to use a transmit beam that has a different wavelength than the receive beam.
It is an object of the present invention to provide a charge coupled device (CCD) as a wide field of view beacon detector.
It is another object of the present invention to integrate a CCD beacon detector into a telescope that is also used to handle transmit beams and receive beams.
The optical communication terminal of the present invention includes a transmit path along which optical transmit energy passes and a receive path along which received optical energy passes. In addition, a shared path exists in the optical communication terminal along which both the received optical energy and the optical transmit energy pass.
A telescope is provided that is constructed using a housing, a primary mirror placed inside or at one end of the housing and a secondary mirror placed at the other end. The primary mirror directs received beams to a secondary mirror and directs the optical transmit energy out of the telescope in a transmit beam. A secondary mirror is placed confocal with the primary mirror which directs transmit optical energy to the primary mirror. The secondary mirror further directs a received communication beam into the shared path as received optical energy.
An annular mirror is positioned along the shared path. The annular mirror passes the optical transmit energy from the transmit path into the shared path and also passes the received optical energy from the shared path into the receive path.
The optical communication terminal may also include a beacon transmitter that emits a beacon beam used for acquisition. A CCD or other detector may then be provided to detect the beacon beam. Preferably, the secondary mirror is a dichroic element that reflects optical energy at the wavelength of the receive beam (and transmit beam) off of the front surface and that forms a lens which passes and focuses optical energy at the wavelength of the beacon beam. The CCD may then be placed behind the secondary mirror lens to detect the beacon beam.